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1
Put the olive oil in a 10- or 12-inch skillet over medium heat.
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2
Add the onions, a healthy pinch of salt, some pepper, and the thyme.
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3
Cook, stirring occasionally, until the mixture starts to sizzle, a minute or two.
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4
Adjust the heat so that you have to stir only every 5 minutes at most to keep the onions from browning.
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5
They will become progressively softer; do not allow them to brown.
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6
Figure at least 30 minutes total for the onions to cook.
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7
Core the tomatoes, then cut them in half horizontally.
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8
Squeeze and shake out the seeds, then cut the tomatoes into 1/2-inch dice.
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9
Preheat a grill to moderately hot.
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10
When the onions are very soft, almost a shapeless mass, season the fish and grill it, turning once, for a total of about 6 minutes for tuna, 8 to 10 minutes for swordfish; check for doneness by making a small cut in the center of the fish and peeking inside.
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11
(Tuna can be quite rare; swordfish is best cooked to medium, when its interior is still slightly pearly rather than completely opaque.)
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12
While the fish is grilling, stir the olives and tomatoes into the onions and raise the heat a bit; cook, stirring occasionally, until the tomatoes liquefy and the mixture becomes juicy.
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13
Taste and adjust the seasoning.
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14
Serve the fish on a bed of the onion confit.
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15
Chicken with Onion Confit: Serve the onions with grilled skinless, boneless chicken breasts.
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16
Omit the thyme and use a bay leaf instead, or finish the dish with a handful of chopped fresh basil, chervil, or parsley.
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17
Cook some finely chopped aromatic vegetables, like carrots, celery or fennel, and garlic, along with the onions.
